Power brake for motor vehicles

ABSTRACT

A brake power assist unit for motor vehicles, the control valve of which can be electrically actuated by means of an electromagnet which actuates a third sealing seat that allows ventilation of the operating chamber. In order to stabilize the characteristic of the load system formed by the armature of the electromagnet, a sleeve that carries the third sealing seat and the valve body of the control valve by means of constructive modifications of the control valve of the externally actuated brake power assist unit, in particular, during the pressure reduction phase, the invention proposes means that allow a controlled pneumatic pressure compensation between the operating chamber and the vacuum chamber or a reduction in the force to be generated by the electromagnet during the pressure reduction phase, i.e., during the return movement of the third sealing seat from the valve body.

TECHNICAL FIELD

The invention pertains to a pneumatic brake power assist unit for motorvehicles, and more particularly relates to pneumatic brake power assistunits which are electrically actuatable.

BACKGROUND OF THE INVENTION

Such a brake power assist unit and its respective application inregulating the brake pressure is known from DE 4,324,205 A1. In thiscase, the position of a sleeve is controlled by means of a positionregulator that receives signals from a displacement transducer andprevents an excessive destabilization of the load characteristic bycontrolling the current delivered to the electromagnet in such a waythat a stable position of the sleeve is attained. In this brake powerassist unit, a pressure regulator is connected in series with theposition regulator, wherein the output signal of the pressure regulatorcorresponds to a nominal value position for the ancillary positionregulator.

The present invention is based on the objective of proposing measureswhich allow a stabilization of the load characteristic during thepressure reduction phase due to constructive modifications on thecontrol valve of the brake power assist unit that can be externallycontrolled. The invention, in particular, aims to eliminate the need forthe aforementioned costly position regulator as well as the sensorarrangement.

According to the invention, this objective is attained by providingmeans that allow a controlled pneumatic pressure compensation betweenthe operating chamber and the vacuum chamber or a reduction of the forceto be generated by the electromagnet during the pressure reductionphase, i.e., during the return movement of the third sealing seat fromthe valve body.

According to one advantageous additional refinement of the invention, aring that is arranged such that it can be moved relative to the thirdsealing seat is supported on the sleeve by way of an elastic orcompressible element, wherein the edge region of the ring which facesthe valve body is provided with openings and arranged axially offsetrelative to the third sealing seat, namely toward the valve body, in theseparated position.

In another advantageous embodiment of the invention, the valve body isprovided with an additional annular surface that contains radialopenings within the region in which it contacts the third sealing seat.

In another alternative embodiment, the third sealing seat is providedwith axial recesses or depressions that, for example, may be realized inthe form of a corrugated sealing edge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, a partial longitudinal section through one embodiment of thebrake power assist unit according to the invention in the inactivestandby position 7.

FIGS. 2a-d, enlarged representations of a first embodiment of thecontrol valve of the brake power assist unit according to FIG. 1, namelyin the home position, the pressure build-up position, the pressuremaintaining position and the pressure reduction position.

FIG. 3, a second embodiment of the control valve of the brake powerassist unit according to FIG. 1 during a regulated pressure build-up.

FIGS. 4a and 4 b, a third and a fourth embodiment of the control valveof the brake power assist unit according to FIG. 1 during a regulatedpressure build-up.

FIG. 5, a diagram that indicates the function of the electromagnet andthe control valve of the brake power assist unit shown in FIG. 1 and thedependence of the force F_(EM) generated by the electromagnet and thepower requirement of the sleeve on the travel S_(H) of the sleeve,respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The schematically indicated power assist unit housing 1 of the vacuumbrake power assist unit according to the invention is divided into anoperating chamber 3 and a vacuum chamber 4 by means of an axiallymovable wall 2. The axially movable wall 2 consists of a diaphragm disk8 that is deep-drawn from sheet metal and an adjacent flexible diaphragm18 that forms a seal in the form of a roller diaphragm between the outercircumference of the diaphragm disk 8 and the power assist unit housing1.

A control valve 12 that can be actuated by means of an actuating rod 7is accommodated in a control housing 5 that carries the movable wall 2and is guided in the power assist unit housing 1 in sealed fashion. Thiscontrol valve consists of a first sealing seat 15 arranged on thecontrol housing 5, a second sealing seat 16 arranged on a valve piston 9that is connected to the actuating rod 7 as well as an annular valvebody 10 that cooperates with both sealing seats 15, 16 and is guided ina guide part 21 that is arranged in the control housing 5 in sealedfashion, wherein the annular valve body is pressed against the valveseats 15, 16 by means of a valve spring 22 that is supported on theguide part 21. The operating chamber 3 can be connected to the vacuumchamber 4 by means of a channel 28 that extends laterally in the controlhousing 5.

The brake force is transmitted to an actuating piston of a not-shownmain cylinder of the brake system which is arranged on the vacuum sideof the power assist unit housing via an elastic reaction disk 6 thatadjoins the end surface of the control housing 5 as well as a push rod14 that is provided with a head flange 23.

A return spring 26 that is schematically illustrated in the figure andsupported on the end wall of the power assist unit housing 1 located onthe vacuum side holds the movable wall 2 in the idle position shown. Inaddition, a second compression spring or piston rod return spring 27 isprovided. This second spring is indirectly supported on the actuatingrod 7 as well as on the guide part 21, wherein the force of this secondspring ensures a prestress of the valve piston 9 or its sealing seat 16,relative to the valve body 10.

In order to be able to connect the operating chamber 3 to atmosphereduring the actuation of the control valve 12, a channel 29 that extendsapproximately radially is arranged in the control housing 5. The returnmovement of the valve piston 9 at the end of a braking maneuver islimited by a transverse element 11 that adjoins a limit stop 38 arrangedin the power assist unit housing 1 in the separated position of thevacuum brake power assist unit shown in FIG. 1.

FIG. 1 also shows that the valve body 10 contains an annular sealingsurface 44 that cooperates with both sealing seats 15, 16. This sealingsurface is reinforced by means of a metal reinforcing disk 45 and isprovided with several axial openings 19. In addition, the valve body 10contains a radially inner sealing lip 13 as well as a radially outersecond sealing lip 41. These sealing lips tightly adjoin theaforementioned guide part 21 which guides the valve body 10 inside ofthe control housing 5 in the assembled state of the valve body 10, suchthat a pneumatic chamber 17 is limited inside of the control housing 5.The flow channels formed by the openings 19 as well as openings in thesealing surface 44 connect the pneumatic chamber 17 to an annularchamber 43 that is limited by the sealing seats 15, 16, wherein theaforementioned pneumatic channel 29 ends in this annular chamber suchthat the pneumatic chamber 17 formed on the side of the valve body 10which faces away from the sealing surface 44 is continuously connectedto the operating chamber 3 and pressure compensation takes place on thevalve body 10.

Consequently, the previously described arrangement allows a reduction ofthe difference between the response force of the brake power assist unitand the restoring force acting upon the valve piston, such that anincrease in the restoring forces can be achieved at a constant responseforce or a reduction in the response force can be achieved at a constantrestoring force, i.e., an improved hysteresis of the brake power assistunit according to the invention is attained.

In order to initiate an external actuation of the brake power assistunit according to the invention which is not a function of the actuatingrod 7, a third sealing seat 24 that can be actuated by means of anelectromagnet 20 is arranged radially between the first (15) and thesecond sealing seat 16. This electromagnet is preferably arranged in ahousing 25 that is connected without rotational play to the valve piston9, i.e., it can be displaced inside the control housing 5 together withthe valve piston 9. The electromagnet 20 consists of a coil 46 arrangedinside the housing 25 as well as a cylindrical armature 31 that can beaxially displaced and is partially guided in a sealing part 30 thatseals the housing 25. A sleeve that is connected to the aforementionedthird sealing seat 24 is supported on this cylindrical armature andholds the armature 31 in its output position, a compression spring 40 isarranged between the valve piston 9 and the sleeve 32, wherein the thirdsealing seat 24 is axially offset relative to the second sealing seat 16arranged on the valve piston 9, such that a gap exists between the thirdsealing seat 24 and the sealing surface 44 of the valve body 10. In thiscase, a ring 36 that preferably encompasses the sleeve 32 radially isprovided. This ring is supported on the sleeve 32 by way of an elasticor compressible element 37 and axially adjoins a limit stop 51 arrangedon the sleeve 32 (FIG. 2) with its other end such that the ring and thesleeve 32 can be moved relative to one another. The width of the ring 36is chosen such that the distance between its edge region that faces thevalve body 10 and the sealing surface 44 is smaller than the distancebetween the sealing surface 44 and the third sealing seat 24. Theaforementioned edge region is provided with axial openings 39. Thesealing part 30 guided in the control housing 5 adjoins theaforementioned reaction disk 6 by way of a transmission disk 33 and thusallows a transmission of the force introduced via the actuating rod 7onto the reaction disk 6.

In the embodiment of the brake power assist unit according to theinvention which is shown in the figure, electrical switching means 47,48 are provided. These switching means are particularly important duringbraking maneuvers, wherein the electromagnet 20 is triggered in additionto the actuation of the brake by the driver in order to realize a fullbrake application independently of the drivers intentions (so-calledbrake assistance function). In this case, it is particularly importantthat the switching means 47, 48 are actuated during each brakingmaneuver. However, it also must be ensured that the electromagnet 20 isreliably switched off after the externally-assisted braking maneuver iscompleted. The switching means shown preferably consists of amicroswitch 47 that has two switching positions and is arranged on thevalve piston 9 or the housing 25 of the electromagnet 20, as well as anactuating element 48 that actuates the microswitch 47 in the form of atranslational movement. This actuating element is guided in a bore inthe control housing 5 in sealed fashion and cooperates with a limit stoprigidly arranged on the power assist unit housing. This limit stop isdesignated by reference numeral 49 and, for example, is formed by aradial collar of the rear power assist unit housing half. A compressionspring 50 is arranged between the actuating element 48 and the controlhousing 5 such that the end of the actuating element 48 which faces awayfrom the microswitch 47 adjoins the limit stop 50 under a certainprestress.

FIG. 2 shows the individual actuating phases of the control valve 12during its external actuation by the electromagnet 20. In the home orstandby position of the control valve 12 shown in FIG. 2a, the valvebody 10 adjoins the first (15) as well as the second sealing seat 16such that both chambers 3, 4 of the brake power assist unit areseparated from one another and the connection between the operatingchamber 3 and atmosphere is interrupted. The edge region of the ring 36mentioned above with reference to FIG. 1 and the third sealing seat 24are respectively situated at a distance from the sealing surface 44 ofthe valve body 10.

In order to reach the pressure build-up position shown in FIG. 2b, theelectromagnet 20 is supplied with current such that the third sealingseat 24 and the ring 36 initially adjoin the valve body 10 while theelastic element 37 is simultaneously compressed, whereafter the thirdsealing seat and the ring displace the valve body 10 toward the rightagainst the force of the valve spring 22 (FIG. 1) such that a gap isformed between the second sealing seat 16 and the valve body 10 and aventilation of the operating chamber 3 takes place. In this case, thefirst sealing seat 15 takes over the function of the third sealing seat24.

In the pressure-maintenance phase shown in FIG. 2c, the second (16) aswell as the third sealing seat 24 are closed such that no changes in thepneumatic pressure built up in the power assist unit housing can occur.

FIG. 2d shows a controlled pressure reduction phase, during which thesecond sealing seat 16 remains closed, the third sealing seat 24 islifted off the sealing surface 44 and the ring 36 adjoins the sealingsurface 44 with its edge region due to the prestress of the elasticelement 37, i.e., a metered removal of the atmosphere from the operatingchamber 3 by suction and consequently a decrease in the pneumaticpressure built up in the pneumatic brake power assist unit 1 takes placeby way of the aforementioned openings 39 that are partially covered bythe end region of the sleeve 32 at the beginning of the pressurebuild-up phase.

In the second embodiment of the object of the invention which is shownin FIG. 3, an elastic element 137 is vulcanized between the sleeve 32and the ring 36 such that the limit stop 51 mentioned above withreference to FIG. 1 or 2 can be eliminated.

FIG. 4a shows one variation of the known sleeve 132, the sealing seat124 of which cooperates with a modified valve body 110. Within theregion in which it contacts the third sealing seat 124, the valve body110 is provided with an additional annular surface 52 that containsradial openings 53. During the pressure build-up, these radial openingsare closed because the third sealing seat 124 is pressed into thematerial of the valve body 110. A metered pressure reduction is achievedby continuously releasing the openings 53 during the return movement ofthe sleeve 132, i.e., before the third sealing seat 124 entirelyreleases the gap between itself and the valve body 110.

FIG. 4b shows another variation with the known sleeve 232, the edgeregion of which that forms the third sealing seat 224 is provided withaxial recesses or depressions 54. These recesses or depressions may, forexample, be realized in the form of a corrugated sealing edge as shownin the figure. When the third sealing seat 224 is pressed against therubber material of the valve body 210, the third sealing seat 224remains unsealed until it is pressed into the rubber material to such anextent that the axial limitation of the depressions 54 also contacts therubber and consequently produces a tight seal. A metered pressurereduction is attained during the return movement of the sleeve 232 dueto the continuous release of the depressions 54, i.e., before the thirdsealing seat 224 entirely releases the gap between itself and the valvebody 210.

The function of the externally actuated brake power assist unitdescribed in this patent or illustrated in the figures is explainedbelow with reference of the diagram according to FIG. 5 which shows theforce-travel characteristics. This figure shows that the plotsidentified by reference symbols I₁, 1 ₂ and I₃ correspond to threedifferent force-travel characteristics of the electromagnet 20 when itis triggered with a first, a second and a third current value. The plotidentified by the reference symbol II represents the behavior of theload system formed by the armature 31 of the electromagnet 20, thesleeve 32, the valve body 10 and the described springs. The firstsection AB of the characteristic II shows the effect of the relativelyweak compression spring 40 arranged between the sleeve 32 and the valvepiston 9. The force of this compression spring must be overcome beforethe third sealing seat 24 adjoins the sealing surfaces 44 of the valvebody 10. The second section BC shows the increase in the force to begenerated by the electromagnet 20 in order to press the edge region ofthe ring 36 into the material of the sealing surface 44 until the thirdsealing seat 24 and the sealing surface 44 of the valve body 10 cancontact one another in point C, in which the characteristic IIintersects the plot 12. In the third section CD, the edge region of thering 36 as well as the third sealing seat 24 are pressed into thesealing surface 44 while simultaneously displacing the valve body 10against the force of the spring 22 until the second sealing seat 16 isopened and the pressure build-up begins in point D. During theadditional displacement of the sleeve 32, a defined gap between thesecond sealing seat 16 and the sealing surfaces 44 of the valve body 10,and consequently a defined gradient of the pneumatic pressure in theoperating chamber 3, is adjusted. Once a desired pressure value isreached, the current supplied to the electromagnet 20 is lowered suchthat the sleeve 32 is moved back. In the section DC, in which thecurrent supplied to the electromagnet 20 is, for example, lowered to thevalue I₁, and the second (16) and the third sealing seat 24 are closed,the pneumatic pressure maintaining phase shown in FIG. 2c takes place.If the current is further lowered to the value I₂, the sleeve 32 and thering 36 are moved further back until a pressure reduction (FIG. 2d) isinitiated. If the current is further lowered, e.g., to the value I₃, thedistance between the third sealing seat 24 and the sealing surface 44becomes larger in the section CB, wherein the edge region of the ring 36adjoins the sealing surface 44 under the influence of the elasticelement 37. The section CB corresponds to a range in which the loadsystem can be adjusted in stable fashion by changing the currentsupplied to the electromagnet 20 between I₂ and I₃, i.e., a range inwhich a defined gap between the third sealing seat 24 and the sealingsurface 44 of the valve body 10 can be adjusted via the edge region ofthe ring 36 and a defined gradient of the pneumatic pressure in theoperating chamber 3 can be adjusted. In this section, the loadcharacteristic II shows a steeper increase than the force-travelcharacteristic of the electromagnet 20 between the current values I₂ andI₃. The point B, in which the force-travel characteristic I3 intersectsthe load characteristic II and in which the edge region of the ring 36is lifted off the sealing surface 44 of the valve body 10 simultaneouslyrepresents the transition between the stable range and an unstable rangeBA.

What is claimed is:
 1. Brake power assist unit for motor vehicles,comprising: a power assist unit housing, the interior of which isdivided into a vacuum chamber and a operating chamber by means of amovable wall, and with a control housing that carries the movable wall,a control valve that controls a pneumatic differential pressure actingon the movable wall is arranged in the aforementioned control housing,wherein the control valve is actuated by an actuating rod by way of avalve piston as well as independently of the actuating rod by means ofan electromagnet that is connected without rotational play to the valvepiston, wherein the control valve consists of three sealing seats thatare arranged mutually concentrically as well as a valve body thatcooperates with the sealing seats, and wherein the first sealing seat isarranged in the control housing, the second sealing seat is arranged onthe valve piston, and the third sealing seat is arranged on a sleevethat cooperates with the electromagnet, means for controlling pneumaticpressure compensation between the operating chamber and the vacuumchamber during a pressure reduction phase said controlling meansincluding a ring that is displaceable relative to the third sealingseat, wherein said ring is supported on the sleeve by way of an elasticor compressible element, and wherein an edge region of said ring whichfaces the valve body is provided with radial openings and is arrangedaxially offset relative to the third sealing seat.
 2. Brake power assistunit according to claim 1, wherein ring radially encompasses the thirdsealing seat.
 3. Brake power assist unit according to claim 1, whereinring adjoins a limit stop arranged on the sleeve.
 4. Brake power assistunit according to claim 1, wherein the elastic element is vulcanizedonto the sleeve as well as onto the ring.
 5. Brake power assist unit formotor vehicles, comprising: a power assist unit housing, the interior ofwhich is divided into a vacuum chamber and a operating chamber by meansof a movable wall, and with a control housing that carries the movablewall, a control valve that controls a pneumatic differential pressureacting on the movable wall is arranged in the aforementioned controlhousing, wherein the control valve is actuated by an actuating rod byway of a valve piston as well as independently of the actuating rod bymeans of an electromagnet that is connected without rotational play tothe valve piston, wherein the control valve consists of three sealingseats that are arranged mutually concentrically as well as a valve bodythat cooperates with the sealing seats, and wherein the first sealingseat is arranged in the control housing, the second sealing seat isarranged on the valve piston, and the third scaling seat is arranged ona sleeve that cooperates with the electromagnet, means for controllingpneumatic pressure compensation between the operating chamber and thevacuum chamber during a pressure reduction phase wherein the thirdsealing seat is provided with axial recesses.
 6. Brake power assist unitaccording to claim 5, wherein the third scaling seat contains acorrugated sealing edge.